Field of the Invention
The present invention relates to an image forming apparatus, for example, a printer, a copier, a facsimile machine, or a multifunctional apparatus, that utilizes an electrophotographic technique.
Description of the Related Art
In an image forming apparatus that utilizes an electrophotographic technique, a two-component developer including a toner and a carrier is used, and, in the case where a developing sleeve serving as a developer bearing member is rotated for a long time unnecessarily, the toner is likely to deteriorate. The toner that has deteriorated may cause an image defect. Therefore, Japanese Unexamined Patent Application Publication No. 9-34243 discloses an apparatus that refreshes toner by adding new toner while forcibly discharging the toner that has deteriorated.
However, it is better to perform the refreshing of toner described above as less frequently as possible because some toner is consumed in the refreshing of toner. Thus, it is better to suppress the deterioration of the toner. The deterioration of the toner may be suppressed by making the time during which the developing sleeve rotates unnecessarily as short as possible. Therefore, Japanese Unexamined Patent Application Publication No. 2012-128320 proposes an apparatus configured to stop the developing sleeve as long as possible, for example, in a sheet interval.
However, in the case where a fog-removing potential difference is present between a photosensitive drum and the developing sleeve when the developing sleeve serving as the developer bearing member is stopped, the fog-removing potential difference is only applied to a part of the developing sleeve opposing the photosensitive drum. In this case, the toner in the developer is likely to attach to the surface of the part of the developing sleeve in a line shape. This attachment of toner to the developing sleeve may cause a belt-shaped image defect called as a stripe image in the subsequent image formation. Thus, the apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2012-128320 stops the developing sleeve after controlling a developing voltage such that the fog-removing potential difference in the sheet interval is smaller than in image formation. The fog-removing potential difference will be also simply described as Vback.
In addition, an image forming apparatus, for example, of a type sharing a developing power source that simultaneously applies a developing voltage from a common developing power source to image forming units of respective colors of yellow, magenta, cyan, and so forth arranged in order in a movement direction of a recording material or an intermediate transfer belt that is used for reducing the cost is known nowadays.
In the case of the image forming apparatus of the type sharing a developing power source, the developing sleeve of the image forming unit of each color is configured to be individually controllable, and is rotated unnecessarily as less frequently as possible in order to suppress the deterioration of toner. At the start of image formation, the rotation of the developing sleeve is started from the image forming unit of yellow, followed by other image forming units in order, and, at the end of the image formation, the rotation of the developing sleeve of the image forming unit of yellow is stopped first, also followed by other image forming units. In this case, the developing voltage starts being applied in the image forming unit for yellow at the start of the image formation, and thus the developing voltage may be applied in the image forming units for magenta and cyan before respective developing sleeves starts rotating, i.e. when the developing sleeves are stopped. Meanwhile, the application of developing voltage is stopped when the image formation by the image forming unit for cyan is finished. This means that the developing voltage remains being applied to the image forming units for yellow and magenta even after the developing sleeves thereof are stopped. That is, in an image forming apparatus of the type sharing a developing power source, toner has been likely to attach to the developing sleeve because the fog-removing potential difference is applied also at the start and end of the image formation even though the developing sleeve is stopped.